/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlinePartyInterface.h
Leave an existing party All existing party members notified of member leaving (see FOnPartyMemberLeft)
true if task was started
bool LeaveParty ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & LocalUserId, const [FOnlinePartyId](API\Plugins\OnlineSubsystem\FOnlinePartyId) & PartyId, bool bSynchronizeLeave, const [FOnLeavePartyComplete](API\Plugins\OnlineSubsystem\Interfaces\FOnLeavePartyComplete) & Delegate )